Fourier Transform Infra red Spectrometry


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Working principle and Instrumentation of FT-IR Spectroscopy


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INTRODUCTION FT-IR stands for Fourier Transform Infra red. IR Spectroscopy is one the most powerful analytical techniques used for qualitative analysis of different kinds of materials. In IR Spectroscopy, IR Radiation is passed through a sample. Some of the infrared radiation is absorbed by the sample and some of it is passed through (Transmitted). The resulting spectrum represents the molecular absorption and transmission, creating a molecular fingerprint of the sample. Like a finger print no two unique molecular structures produce the same infrared spectrum. This makes infrared spectroscopy useful for several types of analysis .

Need for FT-IR:

Need for FT-IR The original infrared instruments were of the Dispersive type. These instruments separated the individual frequencies of energy emitted from the infrared source. This was accomplished by the use of a prism or grating. The Detector measures the amount of energy which is transmitted by the sample. This results in a spectrum which is a plot of wavelength Vs Transmittance.

FT-IR is preferred over dispersive or filter methods of infrared spectral analysis for several reasons::

FT-IR is preferred over dispersive or filter methods of infrared spectral analysis for several reasons: It is a non-destructive technique It has high speed It is mechanically simple Data collection is easier No need for external calibration Stray light does not affect the detector, since all signals are modulated


THEORY If we regard light as a wave, then we accept the fact it has a wavelength. If two beams of light of the same wavelength are brought together in phase, the beams reinforce each other and continue down the light path. However. If the two beams are out of phase, destructive interference takes place. This is at a maximum when the two beams of light are 180 degrees out of phase. Advantage of this fact is taken in Fourier Transform Spectrophotometer .

Constructive and Destructive Interference:

Constructive and Destructive Interference


INTERFEROMETR To overcome the Limitations of dispersive infrared spectrophotometer, Fourier transform spectrophotometer uses a simple optical device called “INTERFEROMETR”, generally called as “ Michelson interferometer”. The Interferometer produces a unique type of signal which has all of the infrared frequencies encoded into it. The signal can be measure very quickly. Thus, the time elements per sample is reduced to a matter of a few seconds rather than several minutes.

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STRUCTURE : It consists of Four arms. Usually at right angles to each other with a Beam splitter at their point of intersection. Radiation is separated by a Beam splitter into two perpendicular half-beams of equal intensity that passes down into other arms of spectrometer. At the ends of these arms the two half-beams are reflected by mirror back to the beam splitter. Beam splitter recombine and are reflected together on to the detector.

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If the wavelength and phase of the initial radiation equals with side arms radiation then the two Half-beams are Recombined and they will still in a Phase. Maximum signal is obtained on the Detector. If the mirror in one arm is moved up by one-quarter of a wavelength, then one of the half-beam will be in out of phase with the other signal obtained on the detector is minimized. In practice, the mirror in one arm is kept stationary and the second arm is moved slowly in the direction of beam splitter. The signal produced is called an “INTERFEROGRAM”

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The Frequency of signal is equals to 2(Velocity of the moving mirror) Frequency = Wavelength of the Radiation Therefore the Frequency depends on the Wavelength of Radiation and Velocity of the moving mirror. In practice, mechanically it is difficult to move the reflecting mirror. To regulate the velocity of the moving mirror Helium and Neon laser is used. Analyst requires a Frequency spectrum in order to make identification because the measure signal can not be interpreted directly. A means of Decoding the individual frequencies are required. This can be accomplished via a well known mathematical technique called the Fourier Transformation.

Data process of FT-IR Interferogram to Spectrum:

Data process of FT-IR Interferogram to Spectrum

A Model FT – IR Spectrum:

A Model FT – IR Spectrum Group frequency region – 4000cm-1 to 1500cm-1 Finger print region – 1500cm-1 to 400cm-1


INSTRUMENTATION OF FT-IR Fourier Transform IR Spectroscopy consists of The light Source Interferometer The Sample Detector

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The Source: 1. Incandescent lamp 2. Nernst glower: It is a Hollow rod consisting of rare earth Oxides like Zirconia, Yttria and Thoria 3. Globar source: Rod made of sintered silicon carbide when heated to high temperatures emits IR radiation 4. Mercury arc lamp: Lamp made of Quartz encloses mercury plasma. It is more effective

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FT-IR Spectroscopy

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The Interferometer: It consists of Beam splitter. Beam splitter is made of material which has 50% refractive index. 3 types of Beam splitters 1. For Far IR: Myiar film sandwiched between Halide plate of low refractive index solid is used 2. For Middle IR: Thin film of Geranium or Halide deposited on CsCl or CsBr or KCL or NaCl 3. For Near IR: Thin film of Ferric oxide deposited on Calcium chloride

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The Sample: The Beam enters the sample compartment it may transmitted through or reflected off of the surface the sample. Absorption of the energy id depends on the characteristic of the sample . Solid: Pressed pellet technique Nujol mull technique Liquids: Sample cell made up of Alkali halides.

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The Detector: 1. Pyroelectric detector: It consists of Thin Dielectric flake. Most commonly used pyroelectric is triglycine sulphate . It is a detector of choice for FT-IR. 2. Photoconductivity cell: It consists of a Thin layer of Semi conducting material like Lead sulphide or Lead telluride. Response time is 0.5msec Thermal conductors are not used because of low sensitivity. Modulator: They are required to convert high frequency radiation to low frequency radiation because high frequency radiation is not detected by detector.

Advantages of FT-IR:

Advantages of FT-IR Multiplex advantage Speed Felgett Advantage: All the frequencies are measured simultaneously, most measurements by FT-IR are made in a matter of seconds rather than several minutes. Sensitivity Mechanical simplicity Internally calibrated Helium and Neon laser as an internal wavelength calibration standards. These instruments are self calibrating and never need to be calibrated by the user.


APPLICATIONS Identification of Inorganic and Organic compounds Identification of Mixture samples Analysis of solids, liquids and gasses In remote sensing In measurement and analysis of Atmospheric spectra And can also be used on satellites to probe the space


CONCLUSION Due to the high sensitivity, specificity, speed and mechanical simplicity of FT-IR spectrometry it is preferred over other techniques for analysis.

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